1. Field of the Invention
The present invention relates to a brushless AC generator to be mounted in an agricultural or a construction vehicle or the like.
2. Description of the Related Art
A brushless type AC generator that has no brushes is often mounted in an agricultural or a construction vehicle such as a combine harvester or a bulldozer, because such an AC generator is used in a severely dusty environment and required to be maintenance free. This brushless AC generator is so cold a rotary-magnetic-field-type generator, an example of which is disclosed in EP0734607B1. In such an example, a rotor has a plurality of magnetic poles pieces of that is disposed to rotate inside a stator, and a field coil that is surrounded by the pole pieces and fixed to a frame.
The plurality of the pole pieces is polarized alternately and disposed in a circumference at equal intervals and is fixed to each other by a nonmagnetic ring, such as a stainless ring, to rotate together.
This brushless AC generator has the following drawbacks. That is, the output power per size thereof is smaller than the output power per size of an AC generator that has brushes. This is because the magnetic circuit of the brushless generator has series air gaps, which increase magnetic reluctance and decrease the magnetic flux.
The present invention has been made in view of the above problem of the conventional art and has an object of providing an improved brushless AC generator that can generate an increased output power per size without lowering the reliability thereof.
According to a main feature of the present invention, a brushless AC generator includes a stator having an armature coil, a rotor having a shaft, a frame, a pulley to be driven by a belt, a pulley-side bearing supported by the frame at a portion near the pulley and anti-pulley-side bearing supported by the frame at a portion remote from the pulley. The rotor is comprised of a magnetic pole core having a plurality of alternately polarized magnetic pole pieces, a rotary yoke integrated with a group of the magnetic pole pieces that are polarized in one polarity and fixed to the rotary shaft. A stationary yoke is interposed between the other group of the pole pieces that are polarized in the other polarity and the rotary yoke, and a field coil is fixedly surrounded by the magnetic pole core. The anti-pulley-side bearing has a higher magnetic permeability than the pulley-side bearing. Further, the anti-pulley bearing includes higher silicon content than the pulley-side bearing, and the end surface of the anti-pulley-side bearing faces an axial end of the rotary yoke. Therefore, the magnetic reluctance of the magnetic path between the rotary yoke and the stationary yoke can be reduced by the bearing that has an increased magnetic permeability, so that magnetic flux, which is inversely proportional to the magnetic reluctance of all the magnetic paths, can be increased, thereby, to increase the generator output power.
According to another feature of the invention, the outer ring of the anti-pulley-side bearing includes higher silicon content than the outer ring of the pulley-side-bearing, and the end surface of the anti-pulley-side bearing faces an end surface of the rotary yoke.
Because the pulley-side-bearing bears more weight of the pulley that is driven by a belt, the outer ring of the bearing has to be tightly supported by the frame by means of a spacer member such as a retainer plate. Therefore, the pulley-side-bearing is disposed remote from the rotary yoke or the magnetic path of the rotor. On the other hand, the anti-pulley-side bearing bears less pulley load and need not any spacer member. Therefore, it can be disposed near the rotary yoke so that the magnetic reluctance thereof can be reduced and the output power can be increased if it is used as a portion of a magnetic path. Moreover, a standard type or mass-produced bearing can be used for the pulley-side-bearing that bears heavy weight of the pulley, so that manufacturing cost can be prevented from rising.
According to another feature of the invention, the outer ring of the anti-pulley-side bearing includes 0.9-1.1 weight % silicon. In other words, the inner and outer rings of the anti-pulley-side bearing function as a magnetic path between the rotary yoke and the stationary yoke and rotatably supports the rotary shaft, includes 0.9-1.1 weight % silicon while a standard type carbon chrome steel bearing includes 0.2-0.35 weight %.
If the silicon content decreases from the above range, the magnetic permeability lowers. On the other hand, the machinability lowers and the rate of thermal expansion lowers if the silicon content increases. If the thermal expansion of the shaft lowers during high temperature operation, the fit between the bearing box of the frame, which usually supports the shaft made of a low-priced carbon steel, and the outer ring of the bearing on the side opposite the pulley may loosen and cause wear around the fit. This problem can be solved by a simple way of changing silicon content according to the invention.
According to another feature of the invention, a resinous cylindrical member is disposed between the outer ring of the anti-pulley-side bearing and a bearing box for supporting the outer ring.
Therefore, loosening of the fit between the bearing box of the rear frame and the anti-pulley-side bearing during high temperature operation can be prevented, so that the wear can be prevented and the reliability and durability thereof can be improved.
According to another feature of the invention, the bearing box that supports the outer ring of the pulley-side bearing is comprised of an approximately cylindrical member that includes 0.9-1 weight %, and is integrated with the frame.
Therefore, loosening of the fit between the bearing box of the rear frame and the outer ring of the anti-pulley-side bearing can be restrained during high temperature operation, so that the wear can be reduced and the reliability and durability thereof can be improved.
A cooling passage may be disposed near the outside surface of the bearing box that supports the outer ring of the anti-pulley-side bearing, so that loosening of the fit between the rear frame and the anti-pulley-side bearing during high temperature operation can be restricted. This can reduce the wear and increase the reliability and durability thereof.